This is a Continuation Application of PCT Application No. PCT/JP01/08593, filed Sep. 28, 2001, which was not published under PCT Article 21(2) in English.
This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2000-301469, filed Sep. 29, 2000, the entire contents of which are incorporated herein by reference.
1. Field of the Invention
The present invention relates to a separator for a fuel cell (stack) which is used in a fuel cell, a production process thereof, and a solid polymer fuel cell using the separator for a fuel cell (stack) and comprising a solid polymer with ionic conductivity as an electrolyte.
2. Description of the Related Art
Hereinafter, prior art regarding a solid polymer fuel cell will be described with reference to FIGS. 12 and 13. FIG. 12 is a cross-sectional diagram showing a unit cell of a conventional solid polymer fuel cell, and FIG. 13 is a cross-sectional diagram showing a conventional solid polymer fuel cell stack.
In the fuel cell stack, a number of unit cells 26 shown in a schematic diagram 2 are laminated vertically. Each unit cell 26 comprises a film electrode composite 23 comprising a flat solid polymer film 21 and a flat fuel electrode 22a and an oxidant electrode 22b which are disposed on the opposing surfaces of the solid polymer film 21 so as to form a gas diffusion electrode 22; two separators 24 which are in direct contact with the fuel electrode 22a and the oxidant electrode 22b, respectively; and packing materials 25.
To extract an electric current from the film electrode composite 23, a fuel gas and an oxidant gas which are reaction gases must be fed to the electrodes 22a and 22b, respectively. Further, at the same time, components having a function of a current collector must be present in contact the electrodes 22a and 22b. These components which feed these reaction gases to the respective electrodes 22a and 22b without mixing the reaction gases together and have a function of a current collector are referred to as separators 24.
As the solid polymer film 21, a perfluorocarbonsulfonic acid film or the like is used. Since the solid polymer film 21 also serves to prevent mixing of the reaction gases to be fed to the fuel electrode 22a and the oxidant electrode 22b, its area is generally larger than the area of the electrode.
The separator 24 is preferably made of a material which hardly allows the two types of reaction gases to pass therethrough so as to prevent mixing of the reaction gases. Further, since conductivity is required, a material such as metal or carbon is used. A separator 24 on the fuel electrode 22a constitutes a front surface of a unit fuel and another separator 24 on the oxidant electrode 22b constitutes a rear surface of the unit fuel. The separators 24 and packing materials 25 for sealing the reaction gases are disposed so as to form a unit cell 26. The packing material 25 is also provided so as to prevent mixing of the two types of reaction gases and leakage of the reaction gases to the outside. When such a phenomenon as mixing of the two types of reaction gases and leakage of the reaction gases to the outside occurs, stable electric power generation with high efficiency cannot be achieved.
The unit cell 26 comprises the film electrode composite 23, the two separators 24 which are in contact with the fuel electrode 22a and the oxidant electrode 22b, and the packing materials 25.
In the separator 24, a plurality of breakthroughs each referred to as a manifold 27 for feeding the reaction gases to each unit cell and a number of fuel gas channels 28a and oxidant gas channels 28b which connect the breakthroughs with one another are formed. Thereby, gas channels 28 for feeding the fuel gas and the oxidant gas which are required for a cell reaction to the fuel electrode 22a and the oxidant electrode 22b are formed.
Since an electromotive force obtained in the unit cell 26 is as small as 1 V or less, a plurality of unit cells 26 are laminated together and electrically connected to each other in series so as to constitute a fuel cell stack 29, thereby increasing the electro-motive force. In the stack 29, a cooling plate for cooling the cell is generally provided for each unit cell 26 or each group of unit cells 26. The cooling plates are not shown in the drawings.
After a required number of unit cells 26 are laminated to form the stack 29, they are clamped in the lamination direction by means of such a clamping mechanism as clamping plates, clamping rods, springs, nuts or the like. This is done to secure electrical and thermal contacts and sealability in between the unit cells 26.
Meanwhile, to cause the solid polymer fuel cell to generate electric power, water must also be fed to the solid polymer film 21, in addition to feeding of the reaction gases to the electrodes 22a and 22b. This is because the ion conductivity of the solid polymer film 21 is significantly improved when it absorbs water. Conversely, if water is not fed to the solid polymer film 21, stable electric power generation cannot be achieved.
In a conventional solid polymer fuel cell, the ionic conductivity of a solid polymer film, i.e., the performance of the cell is very sensitive to changes in the flow rates, temperatures and humidities of reaction gasses. Hence, for example, when an abrupt load change occurs, a time lag is produced until the reaction gases settle at a temperature and humidity corresponding to the load, so that the performance of the fuel cell is liable to change and unstable during the time lag. Consequently, to keep the performance of the fuel cell constantly stable, a function to cause the fuel cell to adapt to such an environmental change must be provided in the fuel cell or in a system to cause the fuel cell to generate electric power.
As a method of providing such a function in particular into the body of the solid polymer fuel cell, an invention that separators are made of an expansion graphite having high water absorbability is disclosed in U.S. Pat. No. 5,300,370.
However, the invention has the following problems. That is, on one hand, the separator made of the expansion graphite is water-absorbable and hydrophilic, so that it has such advantages as having excellent dischargeability of water remaining in channels, being capable of conforming to an environmental change such as an abrupt load change, and having low gas permeability. On the other hand, since the expansion graphite is a relatively soft material, compression creep may occur when the solid polymer fuel cell is clamped in the lamination direction by a clamping mechanism and kept in that condition as described above, thereby causing an increase in the pressure losses of the reaction gases with time. In addition, since the film electrode composite 23 and the separator 24 are laminated repeatedly in the solid polymer fuel cell as described above, a large contact resistance between the two types of components lowers the voltage of the cell, resulting in low power generation efficiency.
To overcome the problems of the first known example described above, the following separator is disclosed in Jpn. Pat. Appln. KOKAI Publication No. 9-274926 (second known example). The second known example is a separator in which projections that constitute gas channels formed on the surfaces of the separator are composed of a conductive elastic member.
However, when the separator of the second known example is applied to the foregoing solid polymer fuel cell of the prior art, the separator cannot conform stably to an environmental change such as an abrupt load change, since the ionic conductivity of the solid polymer film, i.e., the performance of the cell is very sensitive to changes in the flow rates, temperatures and humidities of the reaction gasses.
The present invention has been conceived to solve such problems. An object of the present invention is to provide a separator for a fuel cell, the separator being made of an expansion graphite, a production process thereof, and a solid polymer fuel cell using the separator. The separator has excellent dischargeability of water remaining in channels due to its water-absorbability and hydrophilicity, is capable of conforming stably to an environmental change such as an abrupt load change, and can be operated safely and stably for a long time with high efficiency.
To achieve the above object, an invention according to aspect 1 is a separator for a fuel cell which has gas channels to feed a fuel gas and an oxidant gas to gas diffusion electrodes of a fuel cell, prevents mixing of the fuel gas and the oxidant gas, and has a function of a current collector, wherein the separator is obtained by press-molding a carbon resin composite material comprising a mixture of a carbonaceous material powder and a thermosetting resin powder, an expansion graphite sheet, and at least one of an electric potential measuring terminal and a temperature measuring probe, at the time of the press-molding, the carbon resin composite material has pits and projections to form the gas channels formed on at least one surface thereof, and the expansion graphite sheet is formed on the surfaces of the carbon resin composite material including the pits and the projections.
According to the invention according to aspect 1, the following effects can be obtained. That is, since a base of the separator is formed by the carbon resin composite material and the expansion graphite sheet is formed on the surfaces of the carbon resin composite material including the pits and projections to form the gas channels, merits of the expansion graphite sheet can be exploited as they are. More specifically, the surfaces of the separator conform to film electrode composites easily, and since the expansion graphite sheet is water-absorbable, the separator can conform to an environmental change such as an abrupt load change stably. Further, the separator can reduce compression creep to a greater extent than a separator comprising only an expansion graphite, and it can be operated safely and stably for a long time with high efficiency.
In addition, according to the invention according to aspect 1, since the expansion graphite sheet has low gas permeability due to a wet seal effect associated with excellent water absorbability, the weight of the separator can be reduced by reducing the density of the carbon resin composite material as the base.
According to the invention according to aspect 1, with the separator for a fuel cell which has at least either an electric potential measuring terminal or a temperature measuring probe incorporated therein, the voltage or temperature of a unit cell can be constantly monitored easily. This is useful because a solid polymer fuel cell must obtain a stable voltage in all unit cells and it is necessary to monitor the potential or temperature of a specific unit cell so as to protect the fuel cell.